Which Pokémon games first introduced regional forms such as Alolan Raichu?

Pokémon Sun and Moon first introduced regional forms such as Alolan Raichu.

Released for the Nintendo 3DS in 2016, Pokémon Sun and Moon are set in the tropical island region of Alola. They introduced Alolan Forms, alternate versions of familiar Pokémon adapted to Alola’s environment and culture.

Alolan Raichu, for example, is Electric and Psychic type rather than simply Electric type, and it can surf on its own tail. Other Alolan Forms include Alolan Vulpix, Alolan Meowth, Alolan Sandshrew, and Alolan Exeggutor. Some have different types, appearances, abilities, or evolutionary relationships.

Regional forms are different from ordinary alternate costumes or cosmetic variations. They are treated as distinct forms with gameplay effects, and their adaptations reflect the region where they live. Later games introduced additional regional variants, including Galarian Forms in Pokémon Sword and Shield and Hisuian Forms in Pokémon Legends: Arceus.